Enantioselective catalysis using chiral metal complexes provides one of the most general and flexible methods for achieving asymmetric organic reactions. Metallic elements possess a variety of catalytic activities, and permutations of organic ligands or other auxiliary groups directing the steric course of the reaction are practically unlimited. Efficient ligands must be endowed with, for example, suitable functionality, appropriate chirality, a structure capable of differentiating space either electronically or sterically and skeletal rigidity or flexibility.
Among the asymmetric organic reactions catalyzed by chiral transition metal complexes, asymmetric hydrogenation has been one of the best studied, due in large part to the fact that it is the basis for the first commercialized catalytic asymmetric process. See, for example, ApSimon, et al., Tetrahedron, 1986, 42, 5157.
Some of the more interesting of the asymmetric hydrogenation catalysts are those derived from BINAP [2,2'-bis(diphenylphosphino) -1,1'-binaphthyl]. See, for example, U.S. Pat. Nos.: 4,691,037; 4,739,084; 4,739,085; 4,764,629; 4,994,607; and 4,766,227. Unlike the more classical models of chiral (asymmetric) molecules, chirality in the case of the BINAP-type compounds arises from the restricted rotation about the single bond joining the naphthalene rings. Isomers arising from this type of asymmetry are termed atropisomers.
BINAP-based cationic Ru(II) complexes induce high enantioselectivity in catalytic reactions. See Takaya et al., Pure & Appl. Chem., 1990, 62, 1137, and Mashima et al., J. Chem. Soc., Chem. Commun., 1989, 1208.
The preparation of the BINAP-bearing monomeric cationic ruthenium complexes, while not only sophisticated, is time consuming and expensive. Accordingly, it would be advantageous to be able to carry out these enantioselective transformations using more readily prepared catalysts. In rhodium catalyzed asymmetric reactions, in situ methods of preparing the active catalysts are well established. However, such an in situ general method to prepare cationic ruthenium catalysts is not known. See, for example, B. Heiser, et al., Tetrahedron Asymmetry, 2, 51 (1991). The necessity of synthesizing the catalyst in an extra step and complications due to catalyst instability are avoided by in situ catalyst generation.